1. Field of the Invention
The present invention relates to an image forming apparatus having a re-feeding path, and more particularly it relates to an image forming apparatus capable of permitting both-surface recording operation.
2. Background Art
An example of a conventional image forming apparatus having the both-surface recording function is shown in FIG. 1. In FIG. 1, a sheet containing cassette 3 containing a plurality of recording sheets S is removably mounted on a body 2 of an image forming apparatus A. A sheet supply roller 5, and a separating pad 6 urged against the supply roller 5 and adapted to separate the sheets S are disposed at a downstream side of the cassette 3. Further, a pair of upper and lower regist rollers 7 and an image bearing member 9 are disposed in order at a downstream side of the supply roller 5. A scanner unit 10 serves to illuminate laser light onto the image bearing member 9 in response to the information from the outside, thereby forming a latent image.
A process cartridge 11 including the image bearing member 9 comprises various function portions for forming a toner image on the image bearing member 9 through a known electrophotographic process. An image transferring device 12 for transferring the toner image onto the sheet S is disposed below the image baring member 9, and a sheet conveying guide plate 13 and a fixing device 15 and the like are disposed in order at a downstream side of the image transferring device.
At a downstream side of the fixing device 15, there is disposed a sheet reversing unit 6 pivotably mounted on the body 2 via a support shaft 16a. The sheet reversing unit 16 includes a face-up ejector tray 17, a face-down/both-surface switching flapper 19, a both-surface/face-up switching flapper 20, an ejecting path 21, a pair of reversing rollers 22, a reversing feeding path 25 and the like. Further, below a frame 26 of the body 2, there is disposed a re-feeding path 27 which includes an inlet guide 29 connected to a downstream end of the reversing feeding path 25, inlet conveying rollers 30, a lateral regist guide member 31, re-feeding rollers 32, a re-feeding path 33 and the like.
After the toner image is transferred from the image bearing member 9 onto the sheet S supplied from the sheet containing cassette 3 by means of the sheet supply roller 5, the toner image is fixed on the sheet by means of the fixing device 15.
When the both-surface recording operation is performed regarding the sheet S, the latter is directed to the sheet reversing unit 16 via the two flappers 19 and 20. After the both-surface/face-up switching flapper 20 is switched or changed to a both-surface recording mode, the switching operation of the face-down/both-surface switching flapper 19 is permitted.
When a trailing end of the sheet S ejected by the reversing rollers 22 rotated in directions shown by the arrows is detected by a reversing sensor 35, the sheet is sent to the reversing feeding path 25 by rotating the reversing rollers 22 in reverse directions and then is conveyed to the lateral regist guide member 31 by means of the inlet conveying rollers 30. A new toner image is transferred onto a back surface of the sheet S re-fed from the lateral regist guide member 31 by the re-feeding rollers 32, thus completing the both-surface recording.
FIG. 2 shows another example of a conventional image forming apparatus B having the both-surface recording function. In FIG. 2, the arrangement from the sheet containing cassette 3 to the fixing device 15 is the same as that of the apparatus shown in FIG. 1.
An ejection/both-surface switching flapper 37 is disposed at a downstream side of the fixing device 15, and intermediate rollers 39, upper ejecting rollers 40, an ejector tray 41 and the like are disposed in order above the flapper 37.
Further, at a lower both-surface sheet re-feeding area, there is arranged an introduction feeding path 49 including an introduction guide plate 42, a flapper 42a inlet conveying rollers 43, a conveying guide plate 45, a reversing sensor 46 and outlet conveying rollers 47. In addition, a reversing guide is protruded from the body 2 of the apparatus B. A re-feeding path 56 is constituted by the reversing guide 50, a lateral regist guide plate 51, a pair of sheet re-supply rollers 52, a flapper 53, a re-feeding guide 55 and the like.
When the both-surface recording is performed regarding the recording sheet S, the sheet S to a front surface of which the toner image was fixed by the fixing device 15 is conveyed onto the reversing guide 50 through the introduction guide plate 42 and the conveying guide plate 45. The sheet S re-fed by the paired re-supply rollers 52 is sent, through the re-feeding guide 55, to the image bearing member 9, where a new toner image is transferred onto a back surface of the sheet. Thereafter, the toner image is fixed on the back surface of the sheet by the fixing device 15, thus finishing the both-surface recording operation.
However, in the conventional image forming apparatus A shown in FIG. 1, although the sheet reversing unit 16 has the face-up rejecting elements such as the face-up ejector tray 17 and the like, since an ejecting angle of the sheet S on which the toner images were fixed is limited, the sheet reversing unit 16 inevitably protrudes rearwardly from the body 2 of the apparatus greatly.
In this case, when the both-surface recording mode is selected, since the sheet S having one surface on which the toner image was recorded is conveyed until a trailing end of the sheet is detected by the reversing sensor 35, a leading end of the sheet further protrudes from the rear end of the sheet reversing unit 16 rearwardly, thus causing a problem that a large installation space is required for installing the image forming apparatus A.
Further, if the sheet is jammed in the reversing feeding path 25 or the re-feeding path 27, since the sheet reversing unit 16 itself protrudes rearwardly, it is very difficult to remove the jammed sheet by swinging the sheet reversing unit 16 around the support shaft 16a. Thus, a maintenance space 56a must be provided for permitting the removal of the jammed sheet from the rear side of the apparatus, thereby making the compactness of the apparatus difficult.
On the other hand, in the conventional image forming apparatus B shown in FIG. 2, particularly when the fixing device 15 performs a heat or thermal fixing process, since the sheet S passed through the fixing device 15 is curled, the feeding path must be straight as much as possible, or, if the feeding path is curved, a radius of the curvature of the path must be great. In this respect, in the image forming apparatus B, the jamming of the sheet is difficult to occur in the both-surface sheet re-feeding portion; however, since the reversing guide 50 protrudes from the body 2 of the apparatus, it is also difficult to make the apparatus small-sized, like to the aforementioned conventional image forming apparatus A.
Further, in both of the above-mentioned image forming apparatuses A and B, since there are many limitations such as the mechanical accuracy of the both-surface feeding mechanism regarding the body 2, the reliability of connections such as the electrical connection between the both-surface feeding mechanism and the apparatus, the safety in the attachment of the both-surface feeding mechanism and an appearance treatment, and since the attachment of the both-surface feeding mechanism is very troublesome, it was practice to manufacture such image forming apparatuses by the makers. Therefore, there arose problems that the maintenance such as maintenance/inspection, replacement of parts and/or repair in the sheet reversing portion and the both-surface sheet feeding portion was worsened, and two inconsistent requirements (compactness of the apparatus and the wider jam treatment space regarding the jam treatment) had to be satisfied.